Hayflick limit is basically how cells can only divide a certain number of times due to telomeres.
It is related to aging but perhaps not the ultimate blockade to immortality
There seems to be myriad factors involved in programmed cellular death beyond the destruction of telomeres.
http://morgana249.blogspot.com/2014/08/7-biologically-immortal-organisms-faces.html
Moreover there are organisms that keep their telomeres lengthened and therefore they are immortal.
For example certain jelly fish lobsters and plants are immortal.
However being immortal doesn’t prevent them from dying haha.
Predators disease major environment changes can still kill these immortal organisms.
http://www.bbc.com/earth/story/20150622-can-anything-live-forever
So Is it the ultimate blockade to immortality?
From my perspective yes it is the ultimate reason we are mortal. However it wouldn’t prevent us from dying.
And humans who do have lengthened telomeres are humans who probably have .